This invention relates to a pilot multiplexing method and transceiver apparatus in an Orthogonal Frequency Division Multiplexing (OFDM) system and, more particularly, to a pilot multiplexing method and OFDM transceive apparatus in an OFDM system for transmitting transmit data by an OFDM signal through an OFDM scheme, and transmitting pilot data by a spectrum signal, e.g., a CDMA signal, in a frequency band identical with that of the OFDM signal.
FIG. 17 is a diagram of the structure of an OFDM transmitting apparatus in an OFDM (Orthogonal Frequency Division Multiplexing) transmission system. User data (channel data) is input to an OFDM modulator 1 serially at a prescribed bit rate. A serial/parallel converter la converts input data to M-bit parallel data S1 to SM, i.e., frequency-domain data of subcarriers f1 to fM, by a serial-to-parallel conversion. An IFFT unit 1b, which is for transmitting the frequency-domain data S1 to SM by the subcarriers f1 to fM, applies an IFFT operation to this frequency data and converts the data to time-domain waveform data. In order to remove inter-symbol interference, a GI (Guard Interval) insertion unit 1c attaches a part of the tail-end data of the time-domain data to the leading end of the data as a guard interval and outputs the resultant signal as an OFDM symbol. ADA converter Id converts this to an analog signal. An up-converter 2 up-converts the OFDM modulated signal (OFDM signal), which enters from the OFDM modulator 1, to a high-frequency signal, and a transmit amplifier 3 amplifies the high-frequency signal, which is then emitted into space from an antenna.
FIG. 18 is a diagram useful in describing a serial-to-parallel conversion. Here a pilot P has been time-multiplexed to the front end of one frame of transmit data. It should be noted that the pilot P can also be dispersed within a frame, as illustrated in FIG. 19. If the pilot per frame is, e.g., 4×M symbols and the transmit data is 28×M symbols, then M symbols of the pilot will be output from the serial/parallel converter la as parallel data the first four times, and thereafter M symbols of the transmit data will be output from the serial/parallel converter la as parallel data 28 times. As a result, in the period of one frame the pilot can be time-multiplexed into all subcarriers and transmitted four times. By using this pilot on the receiving side, channel estimation is performed on a per-subcarrier basis and channel compensation (fading compensation) becomes possible.
FIG. 20 is a diagram useful in describing insertion of a guard interval. If an IFFT output signal conforming to M×N subcarrier samples (=1 OFDM symbol) is taken as one unit, then guard-interval insertion signifies copying the tail-end portion of this symbol to the leading-end portion thereof. Inserting a guard interval GI makes it possible to eliminate the effects of inter-symbol interference (ISI) ascribable to multipath.
FIG. 21 is a diagram of the structure of an OFDM receiving apparatus in an OFDM transmission system. A down-converter 5 applies frequency conversion processing to a high-frequency signal that has been sent from a transceiving apparatus, the receive signal obtained by the frequency conversion is subjected to AGC amplification in an AFC amplifier 6, the resultant signal is converted to a digital signal by an AD converter 7 and the digital signal is input to an OFDM demodulator 8.
In the OFDM demodulator, a symbol timing detector 8a detects the timing of the OFDM symbol based upon correlation of the guard interval (GI) (PCT/JP01/08488). As shown in (a) of FIG. 22, the guard interval GI is created by copying, to the leading end of M-number of samples of the OFDM valid symbol, a tail-portion of NG-number of samples. Therefore, by calculating the correlation between the receive signal that prevailed one OFDM valid symbol earlier (M samples earlier) and the present receive signal, the correlation value will be maximized at the portion of the guard interval GI, as indicated at (b) in FIG. 22. The timing t0 at which the correlation value is maximized is the start timing of the OFDM symbol. A GI removal unit 8b removes the guard interval that has been inserted based upon the OFDM-symbol start timing and inputs the resultant signal to an FFT operation unit 8c. 
The FFT operation unit 8c executes FFT processing at an FFT window timing and converts the time-domain signal to M-number of subcarrier signals (subcarrier samples) S1′ to SM′. A channel estimation unit 8d performs channel estimation for every subcarrier f1 to fM using the pilot that has been time-multiplexed on the transmitting side, and a channel compensator 8e multiplies the FFT output by channel estimation values CC1 to CCM of each of the subcarriers, thereby compensating for fading.
The channel estimation unit 8d sums, on a per-subcarrier basis, a plurality of symbol's worth of subcarrier components S1′-SM′ of each pilot symbol that is output from the FFT operation unit 8c and calculates the channel estimation values CC1 to CCM of each of the subcarriers based upon the average values. That is, the channel estimation unit 8d estimates the influence exp(jφ) of fading on phase of each subcarrier using the known pilot signal, and a channel compensator 8e multiplies the subcarrier signal component of the transmit symbol by exp(−jφ) to compensate for fading.
A parallel/serial converter 8f converts parallel data (subcarrier components S1′ to SM′), which has been channel-compensated by the channel compensator 8e, to parallel data, inputs this data to a data demodulator (not shown) and decodes the transmit data.
The channel estimation values are obtained from the pilot symbol of a known signal. However, there are instances where an average of several symbols is obtained in order to improve the S/N ratio. With regard to the channel value, since it is known that there is some degree of correlation in both the time direction and subcarrier direction, the average can be calculated in a region that extends in both the time and subcarrier directions. FIG. 23 illustrates an example of frame structure. In this case, the frame extends over eight symbols and averaging is performed two subcarriers at a time (total 2×8=16). The reason for obtaining the channel estimation value by averaging as set forth above is that since each symbol contains noise, the influence of such noise is eliminated by averaging to thereby improve the S/N ratio. If subcarriers are very close in terms of frequency, the channel values are almost the same and therefore no problems are caused by averaging.
With the above-described OFDM communication, only one pilot can be used and it is not possible to cope with a case where use of pilots of a plurality of types is desired. For example, in an arrangement in which the zone of a base station BS is divided into sectors and directional beams are emitted from antennas AT1 to AT3 in respective ones of sectors SC1 to SC3, as shown in FIG. 24, it is necessary to identify mobile stations MS1 to M53 sector by sector. Consequently, it is necessary to use pilots that differ for every sector. With the conventional schemes, however, using pilots of a plurality of types is impossible.
A method that uses orthogonal codes is available as one method of multiplexing pilots of a plurality of types (PCT/JP02/00059). As shown in FIG. 25, this method adopts m (=2) adjacent subcarriers at a time as a set and multiplies a total of 16 subcarrier components of each of the sets in n (=8) pilot symbols by orthogonal codes K0 to K15 shown in FIG. 26. In accordance with this method, pilot signals the number of which is equivalent to the number of orthogonal codes can be multiplexed at the same frequency and same timing.
Problems of the Prior Art
In a conventional OFDM according to the prior art, use is made of guard-interval correlation in order to detect the leading-end timing of an OFDM symbol at initial synchronization. With the prior art, however, a problem is that the correlation characteristic is sluggish and, hence, it is difficult to improve detection precision. A further problem is that this detection precision has an influence on data demodulation performance.
In another example of prior art, a master station transmits a symbol synchronization signal toward a slave station by a CDMA scheme. At the slave station, a CDMA decoder decodes a CDMA receive signal and an OFDM synchronizing circuit transmits an OFDM signal through an OFDM modulator and high-frequency converter in sync with the symbol synchronization signal (JP10-210002A). In accordance with this prior art, timing detection precision can be improved. However, since CDMA transmission is performed only for the sake of synchronization, there are problems in terms of performance and cost.
Further, according to the prior art, the pilots are multiplexed by multiplication using orthogonal code. However, with such prior art, each pilot cannot be demultiplexed until the code is received in its entirety. Consequently, it is not possible to cope with an instance where a momentary channel estimation value is desired at the stage where one or two pilot symbols, for example, have been received. Further, with the method of multiplexing pilots by multiplication using the orthogonal code K0 to K15 of 16 bits, a maximum of only 16 pilots can be multiplexed and there is little degree of freedom.